Thieves dressed in jackets emblazoned with the word "police" boarded a fishing boat and stole 70 gold bars worth an estimated $11.5m (£7m), officials on the Caribbean island of Curacao have said.

The boat's captain was hit on the head in the early-morning assault before the thieves made off with the gold in three cars, police spokesman Reggie Huggins said.

Authorities believe there were at least six men involved in the heist. No arrests have been made.

Mr Huggins declined to say who owned the gold bullion but he said it was a legal shipment that was passing through Curacao and officials had been advised in advance that it was coming as part of "normal security protocols".

He declined to disclose the eventual destination of the metal.

Guards to the port area let the thieves inside a restricted area in the mistaken belief that they were customs officials.

The men's jackets had the word "police" in English, but in Curacao the word would be written in Papiamento, one of the island's three official languages, as "polis".

During the robbery, crew members said the gang wore hoods and masks and made off with the gold in a matter of minutes.

"The crew said it was like a movie operation, very fast," Mr Huggins said.

The captain and three crew members were from the South American country of Guyana, he said.

By its appearance with rust streaks on its white cabin and no visible security, the fishing boat would seem an unlikely place to stash that amount of gold.

A crew member who gave his name as Raymond Emmanuel told the Associated Press that they left Guyana several days ago and arrived early on Friday in Curacao.

Contradicting police, he said they were delivering the gold to a company in Curacao but said he did not know the name of the business.

Mr Emmanuel said the gold was locked away when the thieves boarded the vessel, and neither he nor anyone else on the vessel was armed.

"This is normal," he said. "We never carry arms. Since I started working here, I've transported gold once before, and this is the system."

Curacao, just north of Venezuela, is a popular tourist destination as well as an offshore financial centre, especially for people from South America.